Main menu

Search form

How Thunderstorms Form

You are here

How Thunderstorms Form

The developing, mature, and dissipating stages of a thunderstorm Credit: NOAA

Most thunderstorms form
with three stages: the cumulus stage
when storm clouds form, the mature stage
when the storm is fully formed, and then the dissipating stage when the storm weakens and breaks apart.

The Cumulus StageWhen warm, moist
air moves upward in an updraft, puffy cumulus clouds may form in the
atmosphere. The moisture in the air condenses into water droplets as it
rises. The cloud will continue to grow
as long as warm air from below continues to rise.

There are several ways that
an updraft of warm moist air can form. Sometimes
air is forced up the side of a mountain. Air is also forced upward at weather fronts, where warm and cool air masses
collide. But often, updrafts form without a mountain or front to guide
them – just because warm air rises. Air near the ground heats up during the day as energy from
the Sun heats the ground, which then heats the air. The warmed air rises higher
in the atmosphere because warm air has less mass than cool air, making it
lighter.

The Mature StageAs the cumulus cloud
continues to grow, the tiny water droplets within it grow larger too as more
water from the rising air is added to the droplets. The cloud starts to look
dark and grey as more water is added to it. And the growing droplets that make
up the cloud become heavy. Raindrops start to fall through the cloud when the
rising air can no longer hold them up. Meanwhile, cool dry air flows downward
in the cloud, called a downdraft, pulling water downward as rain. With an
updraft, downdraft and rain, the cloud is now called a cumulonimbus cloud and
the cycling of air up and down is called a thunderstorm cell.

The moving air within the
cloud builds up electric charges as it slides past other air. The build up of
electric charges allow lightning to form, much in the same way that you can create
a spark after shuffling your feet on a carpet. Thunder is the sound that
happens when lightning strikes. It often happens after you see the bolt of
lightning because sound travels more slowly than light.

The Dissipating StageWhen the downdrafts
in the cloud become stronger than the updraft, the storm starts to weaken.
Since warm moist air can no longer rise, cloud droplets can no longer form. The
storm dies out with light rain as the cloud disappears from bottom to top. The
whole process takes about one hour for an ordinary thunderstorm. Severe
thunderstorms like supercells and squall lines are much larger, more powerful,
and last for several hours.